1. Field of the Invention
The present invention relates to an isocyanate-based polymer foam and to a process for production thereof. More particularly, the present invention relates to a novel manner of mitigating the exotherm experienced by an isocyanate-based polymer foam during production.
2. Description of the Prior Art
Isocyanate-based polymers are known in the art. Generally, those of skill in the art understand isocyanate-based polymers to be polyurethanes, polyureas, polyisocyanurates and mixtures thereof.
It is also known in the art to produce foamed isocyanate-based polymers. Indeed, one of the advantages of isocyanate-based polymers compared to other polymer systems is that polymerization and foaming can occur in situ. This results in the ability to mold the polymer while it is forming and expanding.
Generally, an isocyanate-based polymer foam may be produced by reaction a mixture comprising an isocyanate, an active hydrogen-containing compound (i.e., a polyol in the case of polyurethane, a polyamine in the case of polyurea, etc.), a blowing agent, a catalyst and one or more other optional ingredients (e.g., fillers, surfactants, chain extending agents, cell openers, and the like).
One of the conventional ways to produce a polyurethane foam is known as the "one-shot" technique. In this technique, the isocyanate, a suitable polyol, a catalyst, water (which acts as a reactive "blowing" agent and can optionally be supplemented with one or more auxiliary blowing agents) and other additives are mixed together at once using, for example, impingement mixing (e.g., high pressure). Generally, if one were to produce a polyurea, the polyol would be replaced with a suitable polyamine. A polyisocyanurate may result from cyclotrimerization of the isocyanate component. Urethane-modified polyureas or polyisocyanurates are known in the art. In either scenario, the reactants would be intimately mixed very quickly using a suitable mixing technique.
Another technique for producing foamed isocyanate-based polymers is known as the "prepolymer" technique. In this technique, a prepolymer is produced by reacting polyol and isocyanate (in the case of a polyurethane) in an inert atmosphere to form a liquid polymer terminated with reactive groups (e.g., isocyanates). To produce the foamed polymer, the prepolymer is thoroughly mixed with a lower molecular weight polyol (in the case of producing a polyurethane) or a polyamine (in the case of producing a modified polyurea) in the presence of a curing agent and other additives, as needed.
The two major categories of isocyanate-based polymer foams are molded foams and slabstock foams.
Generally, molded foams are produced by dispensing a foamable composition into a mold cavity, closing the mold to define a cavity having the desired shape of the article being produced and allowing the foamable composition to polymerize and expand thereby filling the mold cavity.
Generally, slabstock foams are produced as large buns using a continuous or semi-continuous process. These processes usually involve dispensing the foamable composition into a trough having a open top, side walls and a moving bottom conveyer which serves to translate the foaming mass away from the dispensing point. The product here is typically a foam bun. The bun can be 100 feet long with a cross-sectional face of up to 7 feet by 4 feet.
Not surprisingly, when producing slabstock foam, the exotherm of the foam is a significant safety concern. As is known in the art, the reaction between isocyanate and polyol (i.e., when producing a polyurethane foam) is exothermic liberating a significantly large amount of heat. While the exotherm in a molded foam is manageable because the size of the volume product is relatively small, the exotherm in slabstock foam must be specifically addressed since the product is so large. As used throughout this specification, the term "exotherm", when used in the context of an isocyanate-based polymer foam, is intended to mean heat of reaction experienced by the foam during production. Thus, the term, "maximum exotherm" is intended to the mean the maximum heat of reaction experienced by the foam during production--practically, this may be assessed by measuring the maximum temperature reached the foam (typically in the area of the core) directly after production. In practice, once a threshold temperature is reached (typically 165.degree. C.-175.degree. C. for most open cell slabstock foams and up to 200.degree. C. for most rigid, semi-rigid and low airflow slabstock foams), in the presence of air or oxygen, auto-oxidation of the foam may occur resulting in discoloration (product deterioration) and sometimes fire (damaging and/or destroying the manufacturing facility).
The prior art as attempted to address this exotherm problem in slabstock foams using a number of approaches.
One approach relates to replacement of water as an indirect blowing agent with liquid organic blowing agents having a higher heat capacity--i.e., the liquid organic blowing agent would absorb at least a portion of the liberated heat of reaction. Examples of liquid hydrocarbon blowing agents useful for this purpose include: chlorofluorocarbons (e.g., Freon-11, Freon-12, etc.), chlorofluorohydrocarbons (e.g., Freon-142b, Freon-22, etc.), methylene chloride, acetone, 1,1,1-trichloroethane and the like. One problem with this approach is environmental. Specifically, in the mid-1980's, various government agencies began to scrutinize the use of organic carbon-based compounds such as hydrocarbon-based and halocarbon-based blowing agents in light of studies which revealed the potential damage caused by escape of such compounds to and interaction with the ozone layer surrounding the Earth. As a result, the governments of many countries in the world have instituted legislation which significantly curtails or even prohibits the use of organic carbon-based blowing agents such as hydrocarbon-based and halocarbon-based blowing agents.
Another approach involves the use of liquid carbon dioxide to replace the carbon dioxide produced in-situ during the reaction between the isocyanate and water. A disadvantage of this approach is that it necessitates significantly high capital cost in the manufacturing facility and there are processing problems (e.g., "pin-holes" in the product and/or poor flow characteristics before rise) with the product.
Yet another approach involves trying to rapidly cool a fresh, "hot" bun of foam by drawing cold or ambient air through the bun. A disadvantage of this approach is that the properties of the foam must be tightly controlled to ensure that it has a high open cell content to allowing the airflow to pass through the foam.
Yet another approach involves the use a reduced atmospheric pressure to produced low density foam without an excessive amount of water. A disadvantage of this approach is that it necessitates significantly high capital cost in the manufacturing facility and the use of relatively expensive copolymer polyols to achieve the firmness of the foam which would ordinarily be lost by reducing the amount of water in the foam formulation.
Thus, despite these various prior art approaches there remains a need in the art for a reliable way of reducing the exotherm inherent in the production of isocyanate-based foams, particularly slabstock polyurethane foams. It would be even more advantageous if the exotherm inherent in the production of the isocyanate-based foam could be reduced without the necessitating an increase in the capital cost of the manufacturing facility and/or the use of relatively expensive chemicals in the foam formulation.